The hydrology and geochemistry of groundwater in tropical island aquifers, such as Barbados, Guam and Puerto Rico, are
significantly influenced by tropical climatic conditions. Recharge to these aquifers is the product of regional and local climate
patterns that control rainfall. Oxygen isotopes can be used to estimate the amount and timing of recharge on these islands
because seasonal fluctuations of rainwater oxygen isotopic compositions are related to the amount of rainfall.
The karst aquifers on Barbados, Guam and Puerto Rico have similar rainwater and groundwater oxygen isotopic
compositions. Comparison of groundwater and rainwater oxygen isotopic compositions in the three aquifers indicates that: (1)
recharge occurs by rapid infiltration with little evaporation prior to recharge; and (2) recharge is associated with similar monthly
rainfall thresholds of 190–200 mm. These rainfall thresholds are remarkably similar for three aquifers in different geographic
locations. Differences between the spatial variations of groundwater oxygen isotopic compositions on Barbados and Puerto
Rico can be attributed to the more complex groundwater flow system on Puerto Rico. The surprising similarities of hydrologic
conditions under which recharge will take place can be attributed to similarities in climate and geologic conditions, such as soils
and limestone bedrock, that exist on the three islands. We therefore speculate that similar recharge-rainfall thresholds may be
observed in other tropical karst aquifers.